The coupling of electromagnetic energy into and out of a subsurface region in which the concentration of a substance is to be measured, is described in principle in the commonly owned copending application Ser. No. 09/333,349 filed Jun. 15, 1999 and based upon German Application 19826265, which is hereby incorporated in its entirety by reference.
The substance which can have its concentration measured by this method can be a substance already in the ground, namely, a natural substance, or it can be a substance artificially present at the measurement location, e.g. because it is intentionally introduced like a marker substance or tracer substance, or because it is a contaminant which, for example, can leach from a waste disposal site or dump.
A method of subsurface exploration in which a marker substance is introduced at at least one location in the ground and in which the migration of that substance is measured has been described in the publication "Development of a Single Borehole Process for the Measurement of Horizontal Ground Water Flow" (translated title) by M. Schottler. In this article, a probe which can be introduced into a borehole or well for carrying out the process is described. The probe can comprise a measurement cell which contains two light sources, a lens system and a special video camera. The marker substance can be a fluorescent material which is introduced centrally over an observation range of several millimeters in size and on which the camera is focused. This region lies axially centrally of the measurement range at a location in which the ground water flow passes freely.
The light sources emit light and the marker substance re-emits light shifted in wavelength from the original emitted light and registered by the video camera as illuminated image points. The result is a high contrast point image which is picked up by video techniques and enables the flow direction of the marker substance to be ascertained.
In another probe for insertion into a borehole or well, marketed by GSF Munich, Germany, the movement of a radioactive marker substance can be detected. This system, however, requires the use of short-lived isotopes which must be prepared by irradiation in a nuclear reactor. The process is relatively expensive and, since the marker substances have short lives, the marker substances can be stored only for limited periods of time. As a consequence, carrying out this latter process is problematical.
A probe for lowering into a well for subsurface exploration has also been developed by the University of California and enables sampling of the ground water to be effected, the sample volumes being pumped to the surface. This system does not permit in-situ measurements of concentration or the like.
Finally, mention can be made of a probe developed by the Technical University of Freiberg, Germany which operates utilizing heat pulses. This system can only be used to detect high ground water velocity.